1. Field of the Invention
This relates to receivers for direct sequence spread spectrum and is concerned with code tracking part of the receiving apparatus.
2. Description of the Related Art
The normal implementation of a code tracking device for spread spectrum consists of a so called early prompt late tracking loop and this involves energy detection of the correlated wave form for a correlation that is performed early, normally half a chip early, and a correlation of energy for a chip that is performed late, normally half a chip late. A timing discrimination curve is then generated by subtracting the early from the late and this is used to close a feedback loop to adjust the time, such that the prompt occurs at the correct time.
U.S. Pat. No. 5,402,450 discloses a method and apparatus for estimating the magnitude and phase of multipath induced error in the position in time of the peak of a composite autocorrelation function by synchronising a reference signal to the timing of a received composite signal is synchronised to a reference signal. Time variations between early and late gate timing points of corrupted and uncorrupted autocorrelation functions are detected by employing scanning correlators.
U.S. Pat. No. 5,414,729 discloses a pseudorandom noise ranging receiver which compensates for multipath distortion by making use of multiple correlator time delay spacing. The receiver consists of a sampling circuit, multiple carrier and code synchronising circuits and multiple digital correlators. The sampling circuit provides digital samples of a received composite signal to each of several receiver channel circuits. The synchronising circuits are preferably non-coherent and track any phase shifts in the received signal and adjust the frequency and phase of a locally generated carrier reference signal accordingly. The correlators provide signals which are fed to a parameter estimator from which the delay and phase parameters of the direct path signal from which a range measurement can be corrected.
U.S. Pat. No. 5,347,536 discloses a method of multipath noise reduction for spread spectrum signals in which phase variations among early and prompt lags are detected. Phase and pseudo-range observables are derived from earlier correlation lags and used to generate a multipath-free code cross-correlation shape. Deviations from that shape are used to infer the magnitude of multipath and to generate corrections for pseudo-range observables. Phase variations from early and prompt lags are used to generate corrections for carrier-phase observables.
An aim of the invention is to remove the need to have the early and late correlations taking place at half a chip but specifically, to enable them to be applied at plus or minus one chip. This results in particular difficulties in the normal implementation, in that at plus or minus one chip for a circuit that is operating at the normal timing, the correlation falls to zero and therefore energy detection of the signals will tend to be very noisy and thus a normal implementation would lead to a discrimination curve that is itself very noisy and would lead to a very jittery timing loop.
The apparatus uses spread spectrum signals, which have been Nyquist filtered and therefore it uses signals, the correlation functions of which still has a good slope on it through the zero points and the basic problems are solved by not performing energy detection on the measurements taken or the correlations taken at these intervals but rather, coherently compensating them against a phase reference derived from the prompt signal and thereby reducing the effect of the noise and thus enabling a discrimination signal to be derived which is of a similar signal to noise ratio as would obtained in the more conventional plus or minus half a chip early/late situation.